Semiconductor based devices and circuits consist of active devices, typically transistors, on a silicon wafer surface and a set of conducting wires interconnecting them. This set of wires is typically referred to as the back-end-of-line (BEOL) while the active transistors are referred to the front-end-of-line (FEOL). A complex network of conducting interconnects is desired in order to electrically wire up the large number of devices, thus creating functional circuits. This is accomplished by building a multi-level structure consisting of metallic lines embedded in an insulating dielectric medium. Modern high speed interconnects typically consist of copper (Cu) conductors which are insulated from one another by low dielectric constant (low k) materials. The interconnect structure may consist of as many as fifteen vertically stacked levels of metal with conducting path between levels, called vias. The wires are characterized by their line width and the distance to the nearest neighbor. The sum of this wire width and space is referred to as the pitch. The first few levels of wiring are built at the minimum allowed technology pitch, as determined by lithography. The tight pitch allows for building the densest circuitry, with the higher levels built at multiples of the minimum pitch. This hierarchical structure allows for thick wide lines, also referred to as fat wires, at the higher levels which are typically used for distributing signals and power across the chip. In addition to serving as an electrical insulator the dielectric material provides mechanical support for the multilevel structure.
At present, Cu/low-k multi-level structures are typically formed by dual damascene processing as follows: the dielectric material is deposited as a blanket film, lithographically patterned, and then reactive ion etched (RIE), creating both trenches and vias. The pattern is then coated by a refractory metal barrier such as Ta and TaNx followed by a thin sputtered copper seed layer. The seed layer allows for the electrochemical deposition (ECD) of a thick copper layer which fills up the holes. Excessive copper is removed and the surface is planarized by chemical mechanical polishing (CMP). Lastly, a thin dielectric film also known as ‘cap’ is deposited over the patterned copper lines. This dual damascene process is repeated at each of the higher levels built.
As predicted by Moore's law, semiconductor devices continue to scale down in order to improve device performance and place more transistors on the substrate. The corresponding scaling of the interconnect structure causes an increase the parasitic resistance (R) and capacitance (C) associated with the copper/low-k interconnects. The RC product is a measure of the time delay introduced into the circuitry by the BEOL. In order to reduce the RC delay, low-k and ultra low-k (ULK) materials are used as dielectrics.
A typical type of low-k dielectric is an organo-silicate glass material, also referred to as SiCOH. It consists of cross-linked SiO2-like tetrahedral structures as the backbone and some —CH3 or —H as the terminal groups or side chains to lower polarizability, introduce porosity and reduce volume density. The low-k dielectrics are typically deposited by plasma enhanced chemical vapor deposition (PECVD) process, which mixes the organic precursor for sacrificial porogen (e.g., cyclohexene, and the like) and the matrix precursor for the low-k backbone structure. (e.g., decamethylcyclopentasiloxane, diethoxymethylsilane, dimethyldimethoxysilane, tetramethylcyclotetrasilane, octamethylcyclotetrasilane, and the like). The deposition step is followed by an ultraviolet (UV) curing process to remove the volatile organic porogen which is loosely bonded to the low-k backbone. As a result, porosity is introduced into the low-k dielectrics. In addition, the UV curing process also induces the cross-linking of low-k dielectrics, improving the mechanical strength. However, ULK films are known to be mechanically weaker than their non-porous low-k counterparts. With porosity and reduced dielectric constant comes a reduction in the film's Young modulus. Typical ULK moduli are in the 2-8 GPa range, depending on the degree of porosity, making the ULK films especially susceptible to mechanical stresses during BEOL processing and during chip packaging.
The dielectric film, which caps the top of the damascene metal structure, prevents copper out-diffusion into the surrounding low-k dielectric. From the perspective of performance and reliability, physical and electrical properties of the cap dielectric, such as breakdown voltage, adhesion to underlying metal and dielectrics, hermeticity, internal stress and elastic modulus, are very important. In general, mechanically compressive films with good adhesion to copper help suppress Cu electromigration and provide a mechanically robust structure. Denser compressive films also tend to have a higher breakdown voltage and provide enhanced hermeticity and passivation of the copper lines. A typical dielectric barrier used in advanced semiconductor manufacturing is an amorphous nitrided silicon carbide (SiCNH).
The UV radiation used in the curing process of ULK dielectrics ranges in wavelength from 200 nm to 600 nm and is generated by a UV bulb, illustrated hereinafter with reference to FIG. 1. The radiation can penetrate through the ULK film and damages the SiCNH cap leading to a change in its mechanical stress state from compressive to tensile. This in turn can lead to spontaneous cracking of the porous ULK material above the cap and to poor reliability during chip packaging operation. An existing solution is to replace a conventional single layer SiCHN with a bilayer low-k cap. This solution has two issues: first, the stress state of the cap still changes albeit at a slower rate. The film stays compressive only if the UV cure time is short (<70 sec). Typical ULK cure times are greater than 100 sec. For these longer cure times, the bilayer cap stress state turns tensile; and second, the bilayer cap with a nitrogen-rich SiCNH on the bottom and carbon-rich SiCNH at the top tends to shrink under UV radiation. A rough estimate is about 2% thickness under 70 s of UV cure while the thickness change of high-k (standard) SiCNH is around zero under the same irradiation conditions. This shrinking of the cap is undesirable and can lead to additional mechanical stresses on the BEOL structure.
Referring to FIG. 2, the internal stress change (measured in MPa) of a SiCHN cap film due to exposure to UV during the ULK cure is illustrated. The stress measurement is shown for different cap materials at different UV cure times. More specifically, the stress change due to the exposure to UV during the ULK curing process shows that the internal stress changes from a negative value (compressive stress) to a positive value (tensile stress) as the UV cure time increases. The curve identified as SiCNH high-k represents a conventional deposition process, while the second curve referenced as SiCNH low-k represents the bilayer deposition process. Although the bilayer cap can slow down the stress change rate, the film ultimately turns tensile. (i.e. crosses the y-axis from negative to positive values) This change from compressive to tensile stress can be understood in terms of a bond breaking mechanism in the SiCHN film upon absorption of the high energy UV photons. The resulting broken, also known as dangling, bonds lead to an increase in internal open spaces and a reduced compressive stress. Tensile films are more prone to cracking and loss of adhesion to the underlayer pattern.
FIG. 3 shows the thickness of the bilayer low-k cap shrinked upon exposure to UV radiation. This cap film loses about 2% of its initial thickness due to the loss of bonded hydrogen and carbon groups in the film. Conventional high-k SiCNH does not shrink in thickness upon a similar exposure to UV radiation.
Referring to FIG. 4, the UV-VIS absorption characteristics of typical metal oxide, e.g., ZnO dispersed in double distilled water along-with the associated reactants are shown. Curves 1, 2, and 3 correspond to PVP (polyvinylpyrrollidone used to prevent agglomeration), ZnO nanoparticles, and Zn(NO3)2, respectively. ZnO nanoparticles can be synthesized by way of different types of alcoholic solutions such as methanol, ethanol, propanol, or higher alcohols.
X-ray diffraction, TEM, and EDAX are used to verify the formation of ZnO nanoparticles. The absorption peak for the ZnO nanoparticles is observed at 262 nm, which lies below the bandgap wavelength (shown as the dotted line in FIG. 4) of 388 nm (Eg=3.2 eV) of bulk ZnO. The shift in the absorption edge to lower wavelength is a fundamental property of nanoparticles and is attributed to a widening of the bandgap when particle sizes become small.
Referring to FIG. 5, the nanoparticle size effect on peak absorbance wavelength is illustrated for various particle diameters. The ZnO nanoparticles show peak absorbance at approximately 262 nm when the average particle size is 2.1 nm. The circular point in FIG. 5 indicates the value of the average particle size as obtained from TEM analysis. From the foregoing, it is evident that ZnO nanoparticles exhibit significant confinement effects for particle diameters less than about 8 nm.